State-of-the-art video coding schemes, such as MPEG-4 AVC/H.264 and the upcoming HEVC (High-Efficiency Video Coding), perform coding of image/video content using inter-picture prediction from previously coded/decoded reference pictures, to exploit the information redundancy across consecutive pictures in time. In MPEG-4 AVC video coding scheme, reference pictures in the decoded picture buffer (DPB) are managed either using a predefined sliding-window scheme for removing earlier pictures in coding order from the DPB, or explicitly using a number of buffer management signals in the coded video bitstream to manage and remove unused reference pictures.
Recent developments in the HEVC video coding scheme include the introduction of DPB management using buffer descriptions. Essentially, a buffer description defines the pictures that are retained/included in the DPB instead of defining the pictures to be removed from the DPB. A buffer description is a list of unique picture identifiers indicating all reference pictures that are stored in the DPB. A buffer description is activated at the start of encoding/decoding of a picture. Pictures that are not included in the active buffer description are removed from the DPB. Benefits of buffer descriptions over conventional methods include improved robustness against transmission/delivery losses and simplified handling of non-existent pictures.
Picture referencing structures are often repeated across multiple pictures in a coded video sequence. For example, a low delay coding structure may use a periodic clustering of size four as shown in FIG. 1.
In this example, the picture numbers (0 to 12) indicates both coding order and display/output order of pictures. The pictures P0, P4, P8 and P12 constitute the first temporal layer of pictures and are coded with the highest quality (for example by applying quantization least strongly). Pictures P2, P6 and P10 constitute the second temporal layer and are coded with lower quality than the first layer. Picture P1, P3, P5, P7, P9 and P11 constitutes the third temporal layer and are coded with the lowest quality. In such a periodic clustering structure, pictures the located at the same relative position within their clusters (for example P1, P5 and P9) usually use the same relative picture referencing structure. For example, P5 uses P4 and P2 as reference pictures, while P9 uses P8 and P6 as reference pictures.
In order to accommodate periodic clustering structures such as the above exemplary structure, periodic signaling of buffer descriptions was introduced. A periodic buffer description lists the reference pictures stored in the DPB by specifying the temporal distances/positions of the reference pictures relative to a target picture to be encoded/decoded. In the prior art, a periodic buffer description is signaled once in the picture parameter set (PPS), and then referred to repeatedly in the slice headers of the pictures having the same relative position within a periodic cluster. For example, a periodic buffer description specifying relative positions of {−1, −3} can be used in both P5 to specify {P4, P2} as reference pictures and by P9 to specify {P8, P6} as reference pictures.
FIG. 2 shows an example of the signaling structure of periodic buffer description in the prior art. A plurality of periodic buffer descriptions are specified in the picture parameter sets. Each PPS is identified by a unique “PPS_id” parameter. In a slice header, a “PPS select” parameter identifies the PPS referred to during encoding/decoding of the slice. In the example in FIG. 2, the PPS having PPS_id=0 is selected. A plurality of “BD updating” parameters selects a periodic buffer description out of the predefined buffer descriptions. In the example in FIG. 2, the buffer description BD1 is selected. Additionally, “BD updating” parameters also includes a buffer description modification command. The buffer modification commands assign a picture identifier to a selected buffer element within the selected buffer description. A unique/absolute picture identifier is specified here instead of a relative picture identifier. In the example in FIG. 2, the unique picture PG is assigned to the buffer element BE0 within the buffer description BD1. This modification applies only to the current target slice. To use the same modification in subsequent slices, the slice headers of those subsequent slices shall specify “BD updating” parameters accordingly.